1. Field of the Invention
This invention relates to improved lignin surfactants and their use as dispersants in dyestuffs and inks. More particularly, this invention relates to non-sulfonated lignin surfactants having reduced staining characteristics produced by a three-step process involving (1) methylolating or crosslinking the lignin, (2) blocking the phenolic functions followed by (3) oxidative reactions.
2. Description of the Prior Art
Lignins have long been recognized for their excellent dispersant properties, but their dark color has excluded them from certain uses. For example, lignin may not be used as a textile dye dispersant with many fabrics since this may produce slight staining or darkening of sensitive shades. When water-soluble, sulfonated alkali lignin (from the kraft or soda pulping processes) or lignosulfonate (from the sulfite pulping process) is used as a dye dispersant, it is ball-milled with a dye cake and the mixture is then used for dyeing natural or synthetic fibers. During the dyeing process, some of the lignin can absorb onto the fabric fibers distorting the true color of the dye. The magnitude of the problem depends on the color of the lignin material.
Also, lignins are used in inks and, in particular, printing inks, as disclosed in U.S. Pat. Nos. 2,525,433 and 2,690,973. Unlike textile dye systems which are generally aqueous and require water-soluble dispersants, ink formulations generally rely on organic solvents; and water-solubility may not be required. Therefore, alkali lignins may be employed in their natural, or non-sulfonated, form and lignosulfonates which have been subjected to desulfonation may be used. In standard ink formulations, lignin color is not objectionable. In fact, U.S. Pat. No. 3,503,762 teaches an ink comprising a lignin product as a color constituent thereof rather than as a dispersant, vehicle or water loss reducing agent. However, advance printing and ink technology have increased the use of colored inks which can be deleteriously affected by the dark color of lignin dispersants.
Therefore, a need exists for a light colored lignin dispersant which can be employed in vat or dispersed dyestuffs for textiles, as well as in inks, particularly colored inks.
Lignin in its natural state is almost colorless. The cause of the brown color of alkali lignins (from the kraft and soda pulping processes) and lignosulfonates (from the sulfite pulping process) and the mechanism for the formation of chromophores during the pulping process are not completely known, although numerous suggestions have been made over the years.
The majority of chromophoric structures in alkali lignins and lignosulfonates appear to be some sort of conjugated systems involving quinonoid and side-chain double bonds. These conjugated systems may be cleaved by some oxidative processes or saturated by reductive processes to achieve some reduction of lignin color. Reductive processes change quinones to colorless catecholic structures which, however, are not stable under the influence of oxygen (air) and sunlight.
On the other hand, oxidative processes convert quinonoid structures to colorless aliphatic acids. The oxidative process also causes cleavage of unsaturated carbon-carbon bonds in the propanoid side chains of lignin molecules. By doing so, some extensively conjugated systems (chromophores) are destroyed, resulting in some reduction of lignin color. An advantage of the oxidative process is the fact that colorless end-products in oxidation reactions are stable and chromophores are not reformed thereof. However, uncontrolled oxidative conditions invite random destruction of lignin aromaticity and concurrently give rise to the formation of color bodies. For example, colored quinonoid moieties are produced in lignin by the following oxidative demethylation pathway: ##STR1##
It has been shown that the color of lignin may be reduced to some degree by blocking the free-phenolic hydroxyls in lignin. Several blocking methods have been set forth, such as in U.S. Pat. No. 3,672,817 where the lignin color was reduced as much as 44% by blocking the phenolic hydroxyl with an alkylene oxide or a halogen-containing alkyl alcohol. In U.S. Pat. No. 3,763,139, lignin color was reduced by blocking the phenolic hydroxyl with reagents, such as chloromethane sulfonate, chloromethane phosphonate, 2-chloroethanol and the like. In U.S. Pat. No. 3,769,272, lignin color was reduced by blocking with 3-chloro-2-hydroxypropane-1-sulfonate. And in U.S. Pat. No. 3,865,803, the phenolic hydroxyl was blocked with an agent of the type X(CH.sub.2).sub.n Y, where X is a halogen, activated double bond, epoxide ring, or a halohydrin, Y is a sulfonate, phosphonate, hydroxyl, sulfide, or a secondary or tertiary amine, and (n) is an integer from 1 to 5. Finally, U.S. Pat. No. 4,184,845 discloses a two-step process for reducing the color of lignin by first blocking the phenolic hydroxyl and then oxidizing the blocked lignin by subjecting to air, molecular oxygen or hydrogen peroxide.
Also, in commonly assigned U.S. patent application Ser. No. 438,391 of which the inventor is a co-inventor, a process is disclosed for reducing the color of sulfonated lignins and lignosulfonates by a two-step process of first blocking 80% of the lignin's phenolic functions followed by oxidizing the blocked lignin with chlorine dioxide.
Although each of the above methods gave some reduction of the color of an alkali lignin or lignosulfonate, none have reduced the color to the extent of the process of this invention.